The Loading, Parsing, and Application of CSS: A Key Topic in Web Performance Optimization

The loading, parsing, and application of CSS are critical topics in web performance optimization. Understanding this process is essential for optimizing page load time and enhancing user experience. Below, we’ll explore in detail whether CSS blocks the parsing and rendering of the DOM and the mechanisms behind it.

CSS Loading and DOM Parsing

DOM Parsing

• DOM (Document Object Model) parsing refers to the process in which the browser converts the received HTML byte stream into a DOM tree.
• During the parsing of an HTML document, when the parser encounters a non-blocking resource (such as an asynchronous script), it attempts to download the resource in parallel while continuing to parse the document.

Does CSS Block DOM Parsing?

• CSS itself does not block DOM parsing. That is, the browser continues parsing the HTML and constructing the DOM tree.
• However, CSS blocks DOM rendering and JavaScript execution. This means that while the DOM tree can be constructed, the browser will not perform rendering operations until the relevant CSS has been parsed (i.e., the CSSOM tree has been built). This is to ensure the page is displayed correctly on the screen, avoiding reflow and repaint issues.

When the browser parses the HTML to generate the DOM tree, it also downloads CSS files in parallel and begins constructing the CSSOM (CSS Object Model). The construction of the DOM and CSSOM occurs simultaneously, meaning CSS downloading and parsing do not block the building of the DOM.

CSSOM Tree and Rendering

CSSOM Tree

• CSSOM (CSS Object Model) is a data structure parallel to the DOM that contains all CSS information for the page. The browser uses it to construct the rendering tree.
• When the browser encounters a <link> tag or a <style> tag, it pauses rendering, prioritizes loading and parsing the CSS, and builds the CSSOM tree.

Rendering Tree Construction

• The rendering tree is the result of combining the DOM tree and the CSSOM tree, representing the content that the browser will render.
• The rendering tree cannot be constructed until the CSSOM tree is complete, as the rendering tree requires style information for all DOM elements.

This process can be visualized with a diagram:

Why Does CSS Loading Block JavaScript?

1. Ensuring Accurate Style Calculation: If JavaScript tries to modify the DOM or compute styles while CSS has not been fully loaded and parsed, the style information retrieved by JavaScript may be inaccurate. To prevent this, the browser ensures that all relevant CSS is loaded and parsed before executing JavaScript, so the script retrieves the final styles for DOM elements.

2. Avoiding Reflow and Repaint: If JavaScript were allowed to execute while the CSSOM is incomplete, it could modify the DOM based on incomplete style information. Once the CSSOM is constructed, the browser might need to reflow and repaint elements already rendered, significantly reducing rendering efficiency.

3. Dependency on Parsing Order: During HTML parsing, when the browser encounters a <link rel="stylesheet" href="..."> tag, it immediately starts loading the CSS. When it encounters a <script> tag (without async or defer attributes), it pauses DOM parsing to execute the script. If CSS has not been fully loaded, script execution may depend on incomplete style information. Hence, the browser waits for the CSSOM to be ready before executing the script.

Does Writing CSS in JavaScript Block DOM Rendering?

Whether writing CSS styles in JavaScript blocks DOM rendering depends on how and when the styles are applied. This involves the browser’s rendering process, especially the relationship between JavaScript, CSS, and the DOM. Let’s explore this in detail:

Directly Modifying Element Styles

When CSS styles are applied directly by modifying the style property of a DOM element in JavaScript (e.g., element.style.color = 'red';), such actions typically do not block DOM parsing. However, they may block the rendering process, as the browser must recalculate styles and may trigger reflow and repaint:

• Reflow: Occurs when the size, structure, or certain properties of elements change, requiring the browser to recalculate their positions and sizes.
• Repaint: Occurs when the visual appearance of an element changes (e.g., color or border) without affecting its size or structure, prompting the browser to redraw the element.

Dynamically Inserting <style> or <link> Tags

If JavaScript dynamically adds <style> or <link> tags to the <head>, it can impact rendering:

• Rendering Block: The browser must pause to parse the newly inserted CSS rules before continuing rendering. This can cause significant rendering delays, especially if the CSS file is large or network conditions are poor.
• Performance Impact: Repeated actions, such as inserting <style> tags in a loop, may cause repeated reflows and repaints, severely affecting page performance.

Improper handling of CSS in JavaScript can indeed block or delay DOM rendering, particularly when styles are dynamically generated and applied.

Conclusion

While CSS loading does not block DOM parsing, it does block DOM rendering. CSS loading also blocks subsequent JavaScript execution.

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